Camptothecin, a natural, cytotoxic alkaloid, is a topoisomerase I inbibitor and potent antitumor agent. It was first isolated from the leaves and bark of the Chinese plant, Camptotheca accuminata, by Wall, et al. (J. Am. Chem. Soc., 88 3888 (1966)).
As depicted, camptothecin is a fused ring system, composed of a quinoline (A and B), fused to a pyrrolidine ring (C), fused to an alpha-pyridone ring (D) which in turn is fused to a lactone ring (E). ##STR2## It has an asymmetric carbon at the 20 position making two enantiomeric forms possible. However, the natural occurring compound is found in the "S" configuration as shown above.
Cytotoxic agents are often employed to control or eradicate tumors i.e., they are chemotherapeutic agents. Camptothecin's cytotoxic activity is thought to be directly related to camptothecin's potency as a topoisomerase inhibitor. [For detailed explanations of the topoisomerase function see A. Lehninger, Principles of Biochemistry, 813, Worth Publishers, New York (1982); L. F. Liu, "DNA Topoisomerases," CRC Critical Review in Biochemistry, 1-24, 15 (1983) and H Vosberg, "DNA Topoisomerases: Enzymes that Control DNA Conformation," Current Topics in Microbiology and Immunology, 19, Springer-Verlag, Berlin (1985).] In particular, camptothecin has been shown to be effective in the treatment of leukemia (L-1210) and certain solid tumors in laboratory animals, e.g., see Chem. Rev. 23, 385 (1973) and Cancer Treat. Rep., 60, 1007 (1967).
Unfortunately, in the clinic camptothecin's promise as an effective antitumor agent has not been completely fulfilled. Camptothecin is essentially insoluble in physiologically compatible, aqueous media, and must be modified to make it sufficiently soluble for parenteral administration, a preferred mode for antitumor treatment. It can be made soluble by forming its sodium salt, that is, by opening the lactone with sodium hydroxide (see F. M. Muggia, et al., Cancer Chemotherapy Reports, pt. 1, 56, No.4, 515 (1972)). However, M. C. Wani, et al., J. Med. Chem, 23, 554 (1980), reported that the alpha-hydroxy lactone moiety of ring E is an absolute requirement for antitumor activity.
In the art there are examples of modifications and derivatives of camptothecin prepared to improve its solubility in water. Although many of these derivatives were active in in vitro and in early animal studies using leukemia (L-1210) models, they were disappointing in chronic, animal models involving implanted solid tumors.
Miyasaka, et al., U.S. Pat. No. 4,399,282, discloses a group of camptothecin derivatives substituted at the 7 position with, inter alia, hydroxymethyl and alkoxymethyl. Further, Miyasaka, et. al. in U.S. Pat. No. 4,399,276 discloses camptothecin-7-aldehyde and certain related aldehyde derivatives such as acetals, oximes and hyrazones. More recently, Vishnuvajjala, et al., in U.S. Pat. No. 4,943,579, claimed a series of water-soluble camptothecin derivatives with substituents on the A ring as does Boehm, et al., European Patent application 0 321 122 A2. Other examples of derivatives of camptothecin include Miyasaka, et al., U.S. Pat. No. 4,473,692 and U.S. Pat. No. 4,545,880; and W. Kingsbury, et al., J Med. Chem., 34, 98 (1991). None of these references reported compounds with antitumor activity greater than that of camptothecin itself.
Wani and co-workers reported that 10, 11-methylenedioxycamptothecin is more potent than unsubstituted camptothecin (see M. C. Wani, et al., J. Med. Chem, 29, 2358 (1986) and 30, 2317 (1987)). However, its water solubility is as poor as camptothecin which seriously limits its clinical utility.
We have now found water-soluble analogs of camptothecin with good, topoisomerase I inhibitory activity in vitro, and impressive, antitumor activity in vivo.